In general, a small imaging device installed in a camera-equipped mobile device has disadvantages such as for example field curvature by which a peripheral portion of an image taken by the small imaging device reduced in thickness along its optical axis is blurred, and shading by which the amount of light received from an object is degraded in a peripheral portion of the semiconductor imaging element. In order to correct the above-mentioned field curvature and shading, a semiconductor imaging element is curved by a concave-shaped package, or attached to and curved by an actuator in the conventional imaging device (refer to for example patent documents 1 to 4). The conventional imaging device provided with a semiconductor imaging element curved by the concave-shaped package, or attached to the actuator is increased in thickness along its optical axis, and in weight.
A conventional imaging device provided with a semiconductor imaging element curved without a package or an actuator is disclosed in a patent document 5. In this device, a semiconductor imaging element is fixed, under high temperature, to one side of a substrate different in thermal expansion coefficient from the semiconductor imaging element, and curved, under the normal operation temperature range, by a stress resulting from the difference in thermal expansion coefficient between the semiconductor imaging element and the substrate. A lens and an optical filter made of glass or the like are disposed on the other side of the substrate. The optical filter prevents infrared light from being received by the semiconductor imaging element.
In general, the thermal expansion coefficient of glass is larger than that of silicon for the semiconductor imaging element, and smaller than that of glass epoxy material for the substrate. The semiconductor imaging element is fixed to one side of the substrate under high temperature, while the optical filter is fixed to the other side of the substrate under high temperature. As a result, the optical filter has a center portion curved in a direction toward the lens under the normal operation temperature range, while the semiconductor imaging element has a center portion curved in a direction away from the lens. Here, a reflection-type optical filter is characterized in that a half-value wavelength, i.e., a wavelength at which its transmission decreases to 50%, shifts toward shorter wavelengths when an incident angle of light to the optical filter is increased.    Patent document 1: Japanese Patent Laid-Open Publication 2003-244558    Patent document 2: Japanese Patent Laid-Open Publication 2001-156278    Patent document 3: Japanese Patent Laid-Open Publication 2004-297683    Patent document 4: Japanese Patent Laid-Open Publication 2005-278133    Patent document 5: Japanese Patent Laid-Open Publication 2004-146633